Exercise training has long been recommended as a means of improving effort tolerance and reducing morbidity and mortality in patients with coronary artery disease (CAD). One mechanism of benefit may be through improved endothelial function with enhanced nitric oxide (NO) bioactivity. Such an effect may augment blood flow to exercising skeletal muscle and to the myocardium, and reduce vascular inflammation, platelet activation and adherence which could diminish the risk of thrombosis. In 46 patients with CAD, participating in the Suburban Hospital cardiac rehabilitation program (Protocol 03-H-0086) we detected increases in circulating endothelial progenitor cells (EPCs), which may have the capacity to repair diseased or dysfunctional endothelium. In the last 23 participants (following amendment of the protocol) a subset showed increase in whole blood nitrite - a marker of intravascular NO ? at completion of program. However, not all patients showed EPC mobilization or increased intravascular NO despite compliant program participation and improved effort tolerance. One possibility is that EPCs from patients who fail to derive vascular benefit as evidenced by increased intravascular NO may have different EPC gene expression profiles at baseline or in response to repetitive exercise, resulting in diminished protection of EPCs against oxidant stress with initiation of apoptosis, compared with EPC gene expression in patients who show evidence of EPC mobilization and endothelial repair. The purpose of our study is to 1) Prospectively demonstrate a relationship between EPC mobilization and increased whole blood nitrite as a marker of improved vascular NO bioactivity due to EPC mobilization, and 2) Determine EPC gene expression profiles, with a focus on activation or suppression of genes whose products regulate intravascular redox potential, apoptosis and growth factor and cytokine secretion. We hypothesize that activation or suppression of critical genes in EPCs at baseline or during exercise may determine EPC mobilization, endothelial differentiation and vascular repair potential as evidenced by increased intravascular NO.